Repression of Acetolactate Synthase Activity through Antisense lnhibition

Acetolactate synthase (ALS), the first enzyme in the biosynthetic pathway of leucine, valine, and isoleucine, i s the biochemical target of different herbicides. To investigate the effects of repression of ALS activity through antisense gene expression we cloned an ALS gene from potato (Solanum fuberosum L. cv Désirée), constructed a chimeric antisense gene under control of the cauliflower mosaic virus 35s promoter, and created transgenic potato plants through Agrobacterium fumefaciens-mediated gene transfer. Two regenerants revealed severe growth retardation and strong phenotypical effects resembling those caused by ALS-inhibiting herbicides. Antisense gene expression decreased the steady-state leve1 of ALS mRNA in these plants and induced a corresponding decrease in ALS activity of up to 85%. This reduction was sufficient to generate plants almost inviable without amino acid supplementation. In both ALS antisense and herbicide-treated plants, we could exclude accumulation of 2-oxobutyrate and/or 2-aminobutyrate as the reason for the observed deleterious effects, but we detected elevated levels of free amino acids and imbalances in their relative proportions. Thus, antisense inhibition of ALS generated an in vivo model of herbicide action. Furthermore, expression of antisense RNA to the enzyme of interest provides a general method for validation of potential herbicide targets.